The various situations requiring elastic materials have led to the development of a wide range of natural and synthetic rubbers. Many of the more demanding situations have required blends of these rubbers to provide the proper mix of characteristics. For example, vehicle tires often include styrene-butadiene rubber (SBR), which is the most common synthetic elastomer, polybutadiene (BR), and even natural rubber. The characteristics usually associated with natural rubber, i.e., abrasion resistance, resilience, good high- and low-temperature performance, and tear strength are ideal for tires and similar applications, which experience great punishment.
However, other environments have less demanding strength requirements, but make other strict demands on elastomers. For example, in the clothing industry, elastomers used for form fitting clothing have a unique set of requirements. These include a low stretch modulus, high dimensional stability (to retain the article's shape), low permanent set (to avoid losing the snug fit of a garment), and tear resistance (to avoid tearing while being punctured by the sewing needle). These demands are compounded, for example, when the garment is swimwear. In this area, in addition to the clothing fit requirements, the garment may be exposed to large amounts of sunlight, chlorine from pool water, salt-water, and oils from body perspiration and sun protection lotions.
A common choice of elastomer for clothing elastication purposes is natural rubber (cis-1,4-polyisoprene). It provides excellent elongation properties, can be made soft, has very good tear resistance and is strong. However, it is severely deficient in resistance to sunlight, oils, or chlorine. A common synthetic substitute for natural rubber in clothing is Neoprene tape, which has excellent resistance to oil, ozone, abrasion and solvents. Unfortunately, the neoprene is not as elastic as the natural rubber, and it takes a permanent set when it stretches that can range up to 25%, which greatly distorts a garment. Neoprene is also much more expensive than natural rubber and has a lower yield due to its higher specific gravity.
In recent years, a new type of elastomer has become available, namely epoxidized natural rubber (ENR). ENR is usually produced by the chemical modification of natural rubber latex with peroxycarboxylic acids. A key advantage gained by this modification is increased resistance to swelling by hydrocarbon oils and solvents. ENR also has excellent tensile strength and fatigue properties. In addition, a high degree of reinforcement may be obtained with silica fillers, even in the absence of a coupling agent. However, the surface characteristics (i.e., the look and feel) of the epoxidized natural rubber do not match those of natural rubber, making epoxidized natural rubber a less than ideal choice for garments. Epoxidized natural rubber is also more difficult to sew than natural rubber, partly because of a tendency to tear due to the sewing needle. ENR also has an undesirably high permanent set.
To take advantage of the benefits of both the natural rubber and the epoxidized natural rubber, a hybrid would be ideal. However, it has been found that the specific interactions between the hydrogens of isoprene units (in the natural rubber) and the oxirane oxygens of epoxidized isoprene moieties (in the ENR), are weak. Previous tests have shown that the two materials do not mix well. Without proper uniformity in the attempted blends, it has been difficult to form a blend that has consistent properties needed for applications such as vehicle tires or footwear. In Japanese patent application 1992-126737, a composition of ENR and natural rubber is disclosed, although large percentages of carbon black and oils are necessary to produce the tire treads disclosed therein.
It is thus an object of the invention to provide a composition for use in elastication of garments that has the superior qualities of both natural rubber and epoxidized natural rubber.
It is another object that the composition have high chlorine, salt-water, and oil resistance; a low permanent set; snug gather; and resistance to sunlight exposure.
It is a further object of the invention to provide a composition that attains the desired properties, while being cost-efficient.